1. Field of the Invention
The subject of the invention is press tool.
2. Discussion of the Background
The manufacture of mechanical parts by pressing of powders to give a compact blank, followed by fritting, can involve the use of uniaxial compression including a die consisting of a housing in which the powder is poured, and a punch which is engaged in the housing to produce the compression of the powder or, as a variant, a pair of punches which are engaged in two opposing ends of the housing in two opposite directions. These presses operate at relatively high rates. They have numerous applications: they can concern metal or ceramic mechanical parts such as gears, magnets, nuclear fuel pellets, etc.
This type of method does, however, have disadvantages. One of the most substantial appears when the compressed part is removed from the mould by gradually taking it out of the housing an axial push movement of the punch. The compression has produced radial stresses in the part, which are released as it is removed from the housing, producing a radial expansion. The risks of damage to the part by cracking or breaking are frequent in the orifice of the housing, between the portions which are still stressed and the portions which have suddenly been released, in which stress concentrations appear. Various methods have been used to improve the quality of the parts. One may mention the use of lubricating additives or binders in the powders, or the choice of particular compression sequences by the punches; but additives impair fritting since they are volatile and can be polluting, and the second methods reduce production rates substantially. These two groups of methods also remedy other faults to some degree, such as insufficient cohesion of the material after compression.
Other methods consist in giving the orifice of the housing of the die a bevel or a connection radius to prevent a sudden transition between the stressed state and released state for the part whilst it is being removed from the mould, but this method is effective only with well-determined orifice profiles which are specific to each variety of part, rendering it difficult to implement.
Still other methods consist in adding in the die tubes made of rubber or other flexible materials which facilitate removal from the mould and are then sacrificed, but this is also costly.
Finally, another type of method, described for example in document U.S. Pat. No. 7,128,547, consists in dividing the die into sectors which are assembled during the compression phase and then separated so as to release the residual compression stresses simultaneously for the entire part. Embodiments of such methods often do not include any means to retain the die sectors once the die has been untightened, making them unfit for automation. Others include a mechanism for controlling the movements of the sectors enabling the method to be automated, but they are complex, involving the use of actuators of the sectors, and they do not truly guarantee that the sectors are satisfactorily contiguous when the powder is poured, a necessary factor for satisfactory manufacture.
A variant of this design consists in tightening the die using springs, an external pressure or any other means to reduce its diameter during pressing; it is described in documents EP-A-1 602 473, U.S. Pat. No. 5,694,640 and in the article by Holownia “Balanced die method for metal powder compaction”, published in Powder Metallurgy, vol. 39, no3, Money Publishing. The tightening is stopped after the pressing, enabling the die to expand in order to reduce the extraction friction of the formed part, and thus facilitate mould-removal. The technical problem is slightly different, and these methods do not help improve the transition of the stresses when the die is removed between the portion removed from the mould and the portion still contained in the die. It should also be observed that, in these examples, the centripetal pressure is applied only to the centre of the die, whereas the edges are held rigidly in the device and therefore have no flexibility.